1. Field of the Invention
The present invention relates to a laser beam source control method and a unit, an exposure method and an apparatus, and a device manufacturing method. More particularly, the invention relates to a laser beam source control method and a laser beam source control unit that control a laser beam source whose oscillation frequency is variable, an exposure method using the laser beam source control method and an exposure apparatus comprising the laser beam source control unit, and a device manufacturing method using the exposure method.
2. Description of the Related Art
Conventionally, exposure apparatuses such as a so-called stepper and a so-called scanning stepper have been used in a photolithographic process for manufacturing a semiconductor device (integrated circuit) or the like.
In recent years, the level of integration in the integrated circuit (semiconductor device) has become extremely increased, and the device rule (practical minimum line width) of an exposure apparatus has become finer accordingly. An excimer laser or a solid-state laser, which has an oscillation line in a far-ultraviolet region and has high brightness and high output, is used as such a light source for ultra-fine lithography.
In a conventional exposure apparatus using such lasers as the light source for exposure, the laser that is the light source has been used in an oscillation frequency band (repetition frequency band) of a fixed or a very narrow range. Further, its maximum frequency has been as low as a few hundred Hz to 1 kHz.
However, higher frequency (higher repetition) of laser oscillation frequency has been urged in recent years due to the request for increasing throughput. Further, high sensitive resist such as a so-called chemically amplified resist has been developed as a resist (photosensitive agent) coated on a substrate such as a wafer, and the quality of the resist has thus diversified. Due to the above-described background, in the recent exposure apparatuses, in order to find the optimum exposure conditions or to minimize laser pulse energy and oscillation pulse frequency, the control system of the exposure apparatus decides the laser oscillation frequency and performs laser oscillation, and a settable oscillation frequency range has also expanded with the higher frequency of laser.
In the exposure apparatus, although the control system of the exposure apparatus decides the laser oscillation frequency to oscillate laser as described above, it has recently been found out that a phenomenon could be observed of the output quality of laser (output energy stability, wavelength stability, and variation characteristic of spectrum line width, for example) deteriorating in a particular oscillation frequency band inherent in laser by the influence of suffering shock wave (acoustic wave) or the like that occurs on laser oscillation. Accordingly, inconvenience that the performance of the exposure apparatus deteriorates could occur when the laser is used in the particular oscillation frequency band. Further, since the laser quality varies due to the oscillation frequency, the laser quality is not stable even in the frequency band other than the above-described particular oscillation frequency band.